Shallow trench isolation (STI) is widely adopted in an integrated circuit (IC) to provide electrical isolation between adjacent semiconductor devices formed in a substrate. In a CMOS integrated circuit, STIs are typically formed between like kinds of NMOS or PMOS transistors in a given well or substrate to suppress leakage current between neighboring devices. These are generally referred to as intra-well isolations. STIs may also be formed between NMOS and PMOS transistors formed in separate wells. STIs thus formed are generally referred to as inter-well isolations and are typically used to suppress leakage current between neighboring devices of opposite-type and/or to prevent CMOS latchup from happening, which typically causes device failure.
STIs in a semiconductor device isolate a P-well and an N-well formed in a twin-well CMOS manufacturing process. NMOS and PMOS transistors are then formed in the P-well and N-well, respectively, having source/drain regions and gate regions. Inter-well STIs are formed to separate n+ region of NMOS transistor and p+ region of PMOS transistor. Intra-well STIs are also created to separate n+ region and an adjacent n+ region in the P-well, p+ region and an adjacent p+ region in the N-well, respectively.
STIs may start with forming an STI trench by an etching operation, overfilling the STI trenches with a dielectric such as an oxide, and then removing the dielectric outside the STI trenches. This dielectric helps to electrically isolate the active areas from each other. Typically, forming STI trenches involves creating shallow trenches into semiconductor substrate through an anisotropic etch operation, filling the STI trench with a dielectric material, such as CVD silicon oxide (SiO2), and removing the excess dielectric using a planarization process, such as chemical mechanic polishing (CMP). In general, the inter-well STI isolation spacing (i.e., n+-to-p+) is substantially wider than the minimum intra-well STI isolation spacing (i.e., n+-to-n+ or p+-to-p+) in order to provide desired isolation effects.